I.really like Bluenegel's answer and I hope you follow what he says. I also agree with Glasgowbelle that you may be insecure.
However, I always look to two sides of an issue. Have you ever spoken to your boyfriend about this ex? How does he react? By this I mean, no arguing, just asking him clear questions with no emotional feelings in your tone. You can at some point, tell him that you want to be assured he has no more romantic feelings for this person, and for him to be honest with you. If he gives you what you feel is a straight and honest answer, you can try to live by that and continue to be with him. If he doesnt want to talk about it, makes excuses, or degrades you in any way for asking a simple question, then you can decide if you want to end things. He needs to be as honest as possible, so that there is trust in the relationship.
The only true wisdom is in knowing you know nothing.